1. Field of the Invention
The present invention is related to the polymerization of olefins and, in particular, to a process and catalyst for producing very high melt index polymers.
2. Description of the Prior Art
Peters, et al. in U.S. Pat. No. 2,892,826 describe the polymerization of olefins in the presence of diborane, other hydrides of boron, or lower boron alkyls adsorbed or absorbed on a group 5a oxide.
The use of chromium compounds in the polymerization of olefins is also well-known. U.S. Pat. Nos. 2,825,721 and 2,951,816 teach the use of CrO.sub.3 supported on an inorganic material such as silica, alumina or combinations of silica and alumina, and activated by heating in reducing atmospheres, to polymerize olefins. When, however, a catalyst system of this type is used in techniques such as the well-known particle-form process, the resins produced, while useful in many applications, are unsatisfactory for other because of a deficiency in certain properties such as melt index.
Attempts to improve the properties of polyolefins produced using supported, heat activated chromium oxide catalysts have been made by adding various compounds to the supported chromium oxide prior to the heat activation thereof. Thus, U.S. Pat. No. 3,484,428 discloses adding boranes or alkyl-substituted boranes to such a catalyst, and U.S. Pat. No. 3,780,011 refers to the use of alkyl esters of boron.
It is also known to utilize other chromium compounds as catalysts for the polymerization of olefins. Such compounds include various silyl chromate and poly-alicyclic chromate esters as described, for example, in U.S. Pat. Nos. 3,324,095; 3,325,101; 3,642,749; and, 3,704,287. The use of phosphorus-containing chromate esters in olefin polymerization catalysts have also been disclosed in the aforesaid U.S. Pat. No. 3,704,287; and in U.S. Pat. Nos. 3,474,080 and 3,985,676. Other such catalysts are described in commonly assigned U.S. Pat. No. 4,100,104. Commonly assigned U.S. Pat. No. 4,118,340 of Rekers claims catalysts comprising reaction products of chromium trioxide and an organoborate compound such as triethyl borate.
Certain of these catalytic materials may be employed in Ziegler-type coordination catalyst systems, or in supported form, alone or in conjunction with metallic or orgnometallic reducing agents such as, for example, trialkylaluminum compounds or alkylboranes. Catalyst systems incorporating supported chromium compound catalysts and organometallic reducing agents, particularly organo-aluminum compounds, are disclosed, for example, in U.S. Pat. Nos. 3,324,101; 3,642,749; 3,704,287; 3,806,500; 3,985,676; and in the aforesaid commonly assigned U.S. Pat. No. 4,100,104.
It has also been disclosed in U.S. Pat. Nos. 3,984,351 and 4,049,896, owned by the assignee of the present invention, that the properties of olefin polymers, e.g., melt indices, may be substantially improved by the use of a catalyst prepared by depositing chromium and aluminum compounds on an inorganic support material and heat-activating the supported composition in a non-reducing, preferably oxygen-containing atmosphere, at a temperature of from about 300.degree. C. (572.degree. F.) up to the decomposition temperature of the support. The resulting material, as preferably combined with a metallic and/or non-metallic reducing agent, e.g., a trialkylborane, provides a catalyst system capable of producing polymers having improved flow properties and shear responses in addition to increased melt indices.
For example, U.S. Pat. No. 4,184,028 discloses a process for polymerizing 1-olefins by contacting such olefin with a heat-activated catalyst system having deposited thereon a solid support material having surface hydroxyl groups, an organophosphoryl chromium reaction product of chromium trioxide and a phosphorous compound having the formula: ##STR3## where R is alkyl, aralkyl, aryl, cycloalkyl or hydrogen, but at least one R is other than hydrogen; and an aluminum compound. In accordance with commonly assigned copending application Ser. No. 931,598 filed May 13, 1980, which has been allowed and is now U.S. Pat. No. 4,252,926, the heat activated catalyst system of U.S. Pat. No. 4,184,028 is combined with a hydrogenated alkylborane such as trimethylborane, tripropylborane, tributylborane, trihexylborane, tridodecylborane, tetraethyldiborane, and the like, prepared by reacting the alkyl borane in a solvent with hydrogen gas at a temperature range of 90.degree. to 130.degree. C. and at a pressure ranging from ambient to 1,000 p.s.i. The polymerization product formed by use of this catalyst system is characterized by, inter alia, elevated melt indices, better flow properties and improved shear response.